A motor grader is an articulated vehicle that has a front section and a rear section. The front and rear sections are articulated to one another. The front section has a left front wheel and a right front wheel, the front wheels coupled respectively to opposite ends of an axle so as to be positioned on opposite sides of a fore-aft axis of the motor grader and steerable relative to the axle. The rear section has four wheels, two on each side in a tandem arrangement.
A variety of drive systems may be used to propel a motor grader. For example, there are front-and-rear-wheel-drive (FWD-and-RWD) systems (e.g., all-wheel drive or less than all-wheel drive), front-wheel-drive (FWD) systems, and rear-wheel-drive (RWD) systems. A typical drive system for a motor grader allows operation of the motor grader in a FWD-and-RWD mode, a FWD-only mode (may also be referred to as “precision mode”), or a RWD-only mode. It is known for the RWD system of a motor grader to be a direct drive system between the engine and the rear wheels, while the FWD system is hydrostatically driven (and thus variable).
The drive system of the motor grader may be switched between these modes. The FWD-and-RWD mode generally provides maximum tractive effort useful, for example, in sloppy conditions or other traction-reducing conditions. The FWD-only mode allows operation of the motor grader at reduced speeds (relative to the direct drive system of the rear section) which may be particularly useful for maneuvering in relatively tight areas (e.g., in a cul-de-sac), fine grading, and milling through pavement, to name but a few possible uses. RWD-only mode may be useful where fuel economy is of interest.
It is known for the FWD system of a motor grader to have two hydrostatic transmissions, one for each of the front wheels. In particular, a first hydrostatic transmission drives one of the front wheels, and a second hydrostatic transmission drives the other front wheel. Each hydrostatic transmission has a hydraulic rotary bi-directional variable displacement pump and a hydraulic rotary two-speed motor coupled fluidly in series with the pump (i.e., each motor has two selectable displacement settings). A clutch is positioned between each motor and a respective final drive coupled to a respective front wheel. An electro-hydraulic engagement valve is coupled to each clutch so as to engage the clutch to operatively connect the motor to the final drive when the solenoid of the engagement valve is energized.